Polycarbonate molded articles coated with UV-curable compositions

ABSTRACT

UV-curable coating compositions containing urethane acrylates prepared from hydroxyalkyl acrylates and low-viscosity polyisocyanates having isocyanurate groups, low-viscosity acrylic ester components as reactive diluents, initiators, and optionally solvents and/or other known additives, and their use for coating molded articles of thermoplastic polycarbonates.

This is a divisional application of ser. No. 08/383,328, filed Feb. 3,1995 now U.S. Pat. No. 6,232,360.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the use of special UV-curable coatingcompositions based on urethane-modified acrylate resins for coatingmolded articles of thermoplastic polycarbonates.

2. Description of the Prior Art

Polycarbonate molded articles are used in a variety of ways because oftheir excellent properties, such as transparency, impact or shockresistance and tensile strength. However, the properties of the surfaceof the material, such as its low abrasion and scratch resistance and thelow resistance to solvents, are inadequate for many areas ofapplication.

In the past, very different solutions have been proposed for the removalof this drawback (cf e.g. PCT Applications WO 80/00968 and 86/04592,U.S. Pat. No. 3,968,305, EP-A 0,274,595 and EP-A 0,274,596). Thesepublications are primarily directed to the coating of molded articleswith UV-curing lacquer systems. The processes of these priorpublications, however, have a number of serious disadvantages. Inparticular, the adhesion of the lacquers or the scratch resistanceobtained with the lacquers, as the case may be, is not completelysatisfactory. The coating compositions of the two last-mentioned priorpublications cure only under nitrogen as a deactivating medium.

The coating compositions based on polyacrylate resins having urethanegroups according to EP-A 0,020,344 or according to PCT-Application WO80/100942 also lead to coatings that still do not fully satisfy thepractical requirements with regard to scratch resistance and stabilityto weathering.

The coating compositions according to DE-OS 3,134,157 contain fairlylarge amounts of N-vinyl derivatives of linear or cyclic amides. Adisadvantage of these compositions is the need to carry out the UVirradiation several times in order to obtain good scratch resistance.Furthermore, a low viscosity can be obtained in this case only by usinglarge amounts of N-vinyl pyrrolidone. However, this reduces theresistance to scratching and to solvents as well as the stability toweathering. In addition, if fairly large amounts of N-vinyl pyrrolidoneare used, the risk exists of the partial solution, whitening and stresscracking of the polycarbonate, as long as the coating is not yet cured.N-vinyl-pyrrolidone is also undesirable with regard to job hygiene.

The urethane acrylates according to DE-OS 3,819,627 also are used incombination with N-vinyl compounds, which leads to the disadvantageousconsequences already mentioned.

The urethane acrylates according to DE-OS 4,021,109 are used incombination with tri- to hexafunctional (meth)acrylates as reactivediluents in order to avoid the said disadvantages of the aforementionedsolvents. However, the stability to weathering of the resulting lacquerfilms is insufficient. This becomes recognizable as a loss of adhesioneven after a relatively short time.

Finally, DE-OS 3,318,147 is concerned with special urethane acrylateswithout any connection to the special problem of coating polycarbonatemolded articles.

It is an object of the present invention to provide coating compositionsfor coating thermoplastic polycarbonates which do not suffer from thedisadvantages of the coating compositions of the prior art.

This object may be achieved with the coating compositions of the presentinvention which are described in more detail below. The coatingcompositions to be used according to the invention differ, e.g. from thecoating compositions according to DE-OS 4,021,109 by the use of aselected low-viscosity polyisocyanate component having isocyanurategroups for the manufacture of the urethane acrylates as well as by theuse of mainly linear bis-acrylates as reactive diluent B).

SUMMARY OF THE INVENTION

The present invention relates to UV-curable coating compositionscontaining

A) 20 to 75 wt %, based on the total weight of components A), B) and C),of a reaction product, which is essentially free from hydroxyl andisocyanate groups and is prepared from

A1) one or more hydroxyalkyl acrylates having 2 to 4 carbon atoms in thealkyl group, optionally mixed with up to 30 hydroxyl equivalent %, basedon the total weight of component A1), of other alcohols, and

A2) an polyisocyanate component containing an aliphatic polyisocyanatewhich contains isocyanurate groups, is based on 1,6-diisocyanatohexaneand has an NCO content of 22 to 23.5 wt % and a viscosity at 23° C. of800 to 1400 mPa.s,

B) 5 to 80 wt %, based on the total weight of components A), B) and

C), of a low-viscosity acrylic ester component containing

B1) at least 80 wt % of one or more bis-acrylates having a molecularweight below 350 and based on an alkanediol which may contain etheroxygen atoms and

B2) up to 20 wt %, based on the weight of component B), of one or moreacrylic esters other than B1),

C) 0 to 80 wt %, based on total weight of components A), B) and C), of asolvent or solvent mixture and

D) 0.1 to 10 wt %, based on the total weight of components A), B) andC), of one or more photoinitiators.

The present invention is also directed to the use of the coatingcompositions for coating molded articles of thermoplasticpolycarbonates.

DETAILED DESCRIPTION OF THE INVENTION

The coating compositions to be used according to the inventionpreferably contain 25 to 60 wt % of component A), 10 to 75 wt % ofcomponent B) and 10 to 75 wt % of component C),. wherein thesepercentages are based on the total weight of components A), B) and C).The content of initiator component D) is preferably 1 to 5 wt %, basedon the total weight of components A), B) and C).

Component A) is selected from reaction products of at least 70% byweight of component A1) and optionally up to 30% by weight of componentA2), wherein these percentages are based on the weight of component A).Component A) is “essentially free from hydroxyl and isocyanate groups”which means that in the production of the reaction products, componentsA1) and A2) are used at an NCO/OH equivalent ratio of 0.9:1 to 1.1:1,preferably about 1:1.

Starting compounds A1) are hydroxyalkyl acrylates with 2 to 4 carbonatoms in the hydroxyalkyl group or, less preferably, mixtures of suchhydroxalkyl acrylates with other compounds having alcoholic hydroxylgroups. Suitable hydroxyalkyl acrylates include 2-hydroxyethyl acrylate,the isomer mixture formed by the addition of propylene oxide to acrylicacid, or 4-hydroxybutyl acrylate. 2-hydroxyethyl acrylate is preferred.

Other suitable hydroxyl compounds include mono- to trihydric aliphaticalcohols having molecular weights of 32 to 400, such as methanol,ethanol, n-hexanol, isooctanol, isododecanol, benzyl alcohol, ethyleneglycol, diethylene glycol, propylene glycol, glycerol and alcoholsobtained from these alcohols by alkoxylation.

Component A2) is selected from polyisocyanates having isocyanurategroups, which are based on 1,6-diisocyanatohexane (hexamethylenediisocyanate, HDI) and have an NCO content of 22 to 23.5 wt % and aviscosity at 23° C. of 800 to 1400 mPa.s. These polyisocyanates areobtained in a known manner by the trimerizing HDI until the reactionmixture has an NCO content of 42 to 45, preferably 42.5 to 44.5 wt %,subsequently terminating the reaction and removing unreacted HDI bydistillation to a residual content of less than 0.5 wt %.

The manufacture of component A) by reacting starting components A1) andA2) can be carried out by known methods, optionally by using suitableurethane catalysts. As previously indicated, the reaction is carried outat an NCO/OH equivalent ratio of 0.9:1 to 1.1:1, preferably about 1:1.

Suitable catalysts for this reaction include tin(II) octanoate,dibutyltin dilaurate and tertiary amines such as dimethylbenzylamine.The reaction can be carried out in the absence of the other componentsor also in the presence of reactive diluent component B) and/or solventcomponent C), provided that these components do not contain H atomsreactive with isocyanate groups, i.e., isocyanate-reactive groups.

Urethane acrylate A) can be protected from premature and undesirablepolymerization before the manufacture of the coating compositionsaccording to the invention by the addition of suitable inhibitors andantioxidants, such as phenols and/or hydroquinones, in amounts of 0.001to 0.3 wt % each, based on the weight of the urethane acrylate. A verysuitable inhibitor is phenothiazine, which is used in the examples. Thisinhibitor can be added before, during and/or following the reaction ofcomponents A1) and A2).

Component B) is selected from bis-acrylates of alkanediols, optionallycontaining ether oxygen atoms, provided that the bis-acrylates have amolecular weight of less than 350. These bis-acrylates may be mixed withup to 20 wt %, based on the total weight of component B), of otheracrylic esters of tri- to hexafunctional alcohols. Suitablebis-acrylates include dipropylene glycol diacrylate, tripropylene glycoldiacrylate, 1,4-butanediol diacrylate and 1,6-hexanediol diacrylate.

Suitable acrylic esters of higher alcohols include trimethylolpropanetriacrylate, pentaerythritol tetraacrylate or sorbitol hexaacrylate.Component B) preferably exclusively contains 1,6-hexanediol diacrylate.

Suitable solvents C) include C₅-C₈ aliphatic compounds orcyclo-aliphatic compounds, fluorinated/chlorinated hydrocarbons, C₁-C₆aliphatic or cycloaliphatic alcohols and solvent systems containing

a) 50 to 95 wt % of C₁-C₈ aliphatic alcohols, such as methanol, ethanol,propanol, i-propanol, n-butanol, and i-butanol, and

b) 5 to 50 wt % of aliphatic esters, such as ethyl acetate or butylacetate.

Photoinitiators D) are known and include those described in “Methodender organischen Chemie” (Houben-Weyl), Volume E 20, pages 80 ff, GeorgThieme Verlag, Stuttgart 1987. Preferred are benzoin ethers such asbenzoin isopropyl ether, benzil ketals such as benzil dimethylketal, andhydroxyalkylphenones such as 2-hydroxy-2-methyl-1-phenyl-1-propan-1-one.

The coating compositions according to the invention can be mixed withknown additives E) such as UV absorbers, fillers, pigments, dyes,thixotroping agents, smoothing agents, flatting agents and levellingagents, which can be used in the customary amounts.

Components A), B), C), D) and E) can be mixed by customary methods,e.g., at room temperature or slightly elevated temperatures, preferablywith exclusion of the direct action of light.

The polycarbonate articles can be coated with the coating compositionsaccording to the invention by known methods such as by spraying anddipping processes.

The curing of the coatings is initiated by UV-irradiation, for whichpurpose commercial UV radiation sources are preferably used.

Thermoplastic polycarbonates, their manufacture and their use for themanufacture of molded articles are described for example in “Chemistryand Physics of Polycarbonates”, Polymer Rev. Vol. 9, IntersciencePublishers.

Examples of molded articles of thermoplastic polycarbonates which can becoated according to the invention are all types of articles of dailyuse, wherein abrasion and scratch resistance as well as stability toweathering are required to a particular degree. Examples includeelectrical articles, cover plates, sight screens, safety discs and solidslabs.

In the following examples all parts and percentages are percentages byweight, unless otherwise indicated.

EXAMPLES

The following polyisocyanates are used in Table 1:

Polyisocyanate 1

A polyisocyanate prepared by trimerizing hexamethylene dilsocyanate at60 to 65° C. using trimethylbenzylammonium hydroxide as the catalyst,terminating the trimerization reaction with dibutyl phosphate at an NCOcontent of 43.6 to 44% and removing excess hexamethylene diisocyanate inthe short-path evaporator to a residual content of <0.5%.

NCO content: 23%

Viscosity: 1000 mPa.s (23° C.)

Polyisocyanate 2—Polyisocyanate from mixture M1 of DE-OS 4,021,109

An HDI-based biuret polyisocyanate available form Bayer AG as Desmodur N3200.

NCO content: 23%

Viscosity: 3100 mPa.s (23° C.)

Polyisocyanate 3—Polylsocyanate from mixture M2 of DE-OS 4,021,109

An HDI-based, isocyanurate group-containing polyisocyanate availablefrom Bayer AG as Desmodur N 3300.

NCO content: 22%

Viscosity: 3100 mPa.s (25° C.)

Preparation of urethane acrylates (Component A)

The polyisocyanates set forth in Table 1 were dissolved in1,6-hexanediol diacrylate. After the addition of 0.02% of dibutyltindilaurate and 0.01% of phenothiazine, based on the total weight ofpolyisocyanate and 2-hydroxyethyl acrylate, 2-hydroxyethyl acrylate wasadded while passing through dry air at 50 to 60° C. The temperature wasmaintained until the NCO content had fallen below 0.1%.

TABLE 1 Solutions of urethane acrylates A) in acrylic ester B) Solution2 Solution 3 Starting components (%) Solution 1 (Comp) (Comp)Polyisocyanate 1 (A2) 46.0 — — Polyisocyanate 2 — 46.0 — Polyisocyanate3 — — 47.1 2-Hydroxyethyl acrylate (A1) 29.0 29.0 27.9 1,6-Hexanedioldiacrylate (B) 25.0 25.0 25.0 Viscosity (mPa · s)/23° C.) 4500 6500 8700

The solutions of urethane acrylates according to Table 1 were processedto spraying lacquers according to the formulation given in Table 2 andsprayed in one operation at 20 g/m² on plates (145×65×3 mm) of bisphenolA homopolycarbonate (Makrolon AL 2443, Bayer AG). After a flash-off timeof 3 minutes at room temperature the coatings were cured by means of aUV irradiation plant available from IST (1 mercury high-pressure UVradiation source, 80 W/cm, 1 m/min belt speed).

To determine the scratch resistance, the increase in haziness afterspraying the sample with quartz sand was measured according to DIN52348.

The weathering resistance was tested by means of accelerated weatheringtests (DIN 53231, Xenotest 1200).

To evaluate the adhesion of the coating and its weathering resistance, across-cut test was carried out by scratching a grid of 6 by 6 lines,each at a distance of 1 mm, in the coating, followed by pressing on andrapidly peeling off an adhesive tape.

The test results of the coatings are set forth in Table 3.

TABLE 2 Comparative Comparative Coating Composition (%) Example 1Example 2 Example 3 Solution 1 36.0 Solution 2 36.0 Solution 3 36.01,6-Hexanediol diacrylate (B) 34.9 34.9 34.9 Solvent Mixture* (C) 25.025.0 25.0 2-Hydroxy-2-methyl-1-phenyl-1- 2.0 2.0 2.0 propane-1-one(photoinitiator) Light protective agent** 1.4 1.4 1.4 Light protectiveagent*** 0.7 0.7 0.7 *A solvent mixture containing 27.3% ethanol, 36.3%isopropanol, 27.3% ethyl acetate and 9.1% n-butanol. **Tinuvin 350,available from Ciba-Geigy. ***Tinuvin 292, available from Ciba-Geigy.

TABLE 3 Comparative Comparative Example Example Example 1 2 3 ScratchResistance good good good Loss of adhesion after >3000 <1500 <1500weathering (hours)

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A molded article of thermoplastic polycarbonatecoated with an UV-curable coating composition containing A) 20 to 75 wt%, based on the total weight of components A), B) and C), of a reactionproduct, which is essentially free from hydroxyl and isocyanate groupsand is prepared from A1) one or more hydroxyalkyl acrylates having 2 to4 carbon atoms in the alkyl group, optionally mixed with up to 30hydroxyl equivalent %, based on the total weight of component A1), ofone or more other alcohols, and A2) an polyisocyanate componentcomprising an aliphatic polyisocyanate which contains isocyanurategroups, is based on 1,6-diisocyanatohexane and has an NCO content of 22to 23.5 wt % and a viscosity at 23° C. of 800 to 1400 mPa.s, B) 5 to 80wt %, based on the total weight of components A), B) and C), of alow-viscosity acrylic ester component containing B1) at least 80 wt % ofone or more bis-acrylates having a molecular weight of less than 350 andbased on an alkane diol which may contain ether oxygen atoms and B2) upto 20 wt %, based on the weight of component B), of one or more acrylicesters other than B1), C) 0 to 80 wt %, based on total weight ofcomponents A), B) and C), of a solvent or solvent mixture and D) 0.1 to10 wt %, based on the total weight of components A), B) and C), of oneor more photoinitiators.
 2. The molded article of claim 1 whereincomponent A1) exclusively contains 2-hydroxyethyl acrylate.
 3. Themolded article of claim 1 wherein component A2) exclusively containssaid aliphatic polyisocyanate.
 4. The molded article of claim 2 whereincomponent A2) exclusively contains said aliphatic polyisocyanate.